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In the past several chapters we have learned about electronic structures of atoms and about how atoms combine to form molecules and ionic substances. In everyday life, however, we don’t have direct experiences with atoms. Instead, we encounter matter as collections of enormous numbers of atoms or molecules that make up gases, liquids, and solids. The fact that matter is actually atomic in nature is not really obvious. Although the idea of atoms extends back to the early Greeks, it took a long time for the concept to gain full acceptance, even among physicists and chemists. Once the atomic nature of matter is understood, however, we can readily understand how atoms and molecules give rise to the properties we observe for matter at the macroscopic level. In this chapter we will focus on gases; in Chapter 11 we will discuss liquids and solids.

In many ways gases are the most easily understood form of matter. Even though different gaseous substances may have very different chemical properties, they behave quite similarly as far as their physical properties are concerned. For example, we live in an atmosphere composed of a mixture of gases that we refer to as air. We breathe air to absorb oxygen, O2, which supports human life. Air also contains nitrogen, N2, which has very different chemical properties from oxygen. The atmosphere also contains smaller amounts of other gaseous substances, yet it behaves physically as one gaseous material. The relative simplicity of the gas state affords a good starting point as we seek to understand the properties of matter in terms of its atomic and molecular constitution.

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